Not Applicable.
The present invention relates to a process for the preparation of hydrocarbons from synthesis gas, i.e., a mixture of carbon monoxide and hydrogen, typically labeled the Fischer-Tropsch process. More particularly, this invention relates to the use of silver-modified catalysts for the Fischer-Tropsch process. Still more particularly, the present invention relates to a method for improving the yield of desirable high-carbon-number reaction products by using certain silver-containing catalysts.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. The methanation reaction was first described in the early 1900""s, and the later work by Fischer and Tropsch dealing with higher hydrocarbon synthesis was described in the 1920""s. The first major commercial use of the Fischer-Tropsch process was in Germany during the 1930""s. More than 10,000 B/D (barrels per day) of products were manufactured with a cobalt based catalyst in a fixed-bed reactor. Fischer and Pichler described this work in German Patent 731,295, issued Aug. 2, 1936. Commercial practice of the Fischer-Tropsch process has continued from 1954 to the present day in South Africa in the SASOL plants. These plants use iron-based catalysts, and produce gasoline in relatively high-temperature fluid-bed reactors and wax in relatively low-temperature fixed-bed reactors.
Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream. In particular, there remains a need for catalysts that produce various desired reaction products. The products of the Fischer-Tropsch hydrogenation reaction can range from molecules containing a single carbon to those containing ten, fifteen or more carbons. Single-carbon hydrocarbon molecules are methane, which is the original gas that was converted into synthesis gas in the first step of the two-step process. The multi-carbon products include gasoline, diesel fuel, jet fuel, and various other relatively valuable hydrocarbons that are, notably, liquids at room temperature.
There are continuing efforts to find catalysts that are more effective at producing these desired products. Product distribution, product selectivity, and reactor productivity depend heavily on the type and structure of the catalyst and on the reactor type and operating conditions. It is highly desirable to maximize the production of high-value liquid hydrocarbons, such as hydrocarbons with five or more carbon atoms per hydrocarbon chain.
U.S. Pat. No. 4,619,910 issued on Oct. 28, 1986, and U.S. Pat. No. 4,670,472 issued on Jun. 2, 1987, and U.S. Pat. No. 4,681,867 issued on Jul. 21, 1987, describe a series of catalysts for use in a slurry Fischer-Tropsch process in which synthesis gas is selectively converted to higher hydrocarbons of relatively narrow carbon number range. The catalysts are activated in a fixed-bed reactor by reaction with CO+H2 prior to slurrying in the oil phase in the absence of air. U.S. Pat. No. 4,477,595 discloses ruthenium on titania as a hydrocarbon synthesis catalyst for the production of C5 to C40 hydrocarbons, with a majority of paraffins in the C5 to C20 range. U.S. Pat. No. 4,542,122 discloses a cobalt or cobalt-thoria on titania as a hydrocarbon synthesis catalyst. U.S. Pat. No. 4,088,671 discloses a cobalt-ruthenium catalyst where the support can be titania but is preferably alumina for economic reasons. U.S. Pat. No. 4,413,064 discloses an alumina-supported catalyst having cobalt, ruthenium and a Group 3 or Group 4 metal oxide, e.g., thoria. European Patent 142,887 discloses a silica supported cobalt catalyst together with zirconium, titanium, ruthenium and/or chromium.
Despite the vast amount of research effort in this field, Fischer-Tropsch catalysts that can be used to more efficiently produce the desired hydrocarbon products are desired. There is still a great need to identify effective catalysts for Fischer-Tropsch synthesis; particularly catalysts that provide high C11+ hydrocarbon production, so as to maximize the value of the hydrocarbons produced and thus maximize the process economics. For successful operation on a commercial scale, the Fischer-Tropsch process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, while maintaining high selectivity of the process to the desired products of carbon monoxide and hydrogen. Accordingly, it is desired to provide catalysts that are selective for specified products and also produce useful amounts of the desired products.
Productivity, which is defined as grams of desired product/kg catalyst/hour, is, of course, the lifeblood of a commercial operation. High productivities are essential in achieving commercially viable operations. Accordingly, an important and necessary objective in the production and development of catalysts is to produce catalysts that are capable of high productivity.
U.S. Pat. No. 4,663,355 discloses the addition of gold, silver or copper to a Fischer-Tropsch catalyst comprising cobalt. The ""355 patent purports to show that the addition of gold to the cobalt catalyst reduces the catalyst selectivity for methane in the Fischer-Tropsch reaction. Nevertheless, there is still a need for improvement; particularly, a catalyst is needed that has higher C11 production.
This invention provides a process and system for producing C5+ hydrocarbons, and preferably C11+. A preferred embodiment of the process comprises; contacting a feed stream comprising hydrogen and carbon monoxide with a catalyst in a reaction zone maintained at conversion-promoting conditions effective to produce an effluent stream. A preferred catalyst comprises (a) at least one metal selected from the group consisting of cobalt and ruthenium, (b) a catalyst support comprising alumina, zirconia, silica, aluminum fluoride, fluorided alumina, bentonite, titania, silica-alumina, sulfated zirconia, tungsten doped zirconia, or molecular sieves, and (c) silver. According to a preferred embodiment, the silver is present in an amount of from about 0.01% to about 10% based on the total weight of the catalyst.
It has been discovered that the addition of silver to a cobalt-containing Fischer-Tropsch catalyst significantly improves the C11+ productivity of the catalyst, as compared to the same catalyst in the absence of silver. Likewise, the addition of silver to a cobalt-containing catalyst causes an increase in the olefin/paraffin ratio of the produced hydrocarbons. In particular, it has been found that, for some catalysts, the C11+ productivity is increased by as much as twenty percent.
According to a preferred embodiment of the invention, silver is added in an amount ranging from about 0.01% to about 10% based on the total weight of the catalyst and support. The catalysts of the present invention comprise silver in combination with a cobalt-containing catalyst on a suitable support. Suitable supports are described in detail below. Alternatively, the cobalt catalyst may be used without a support. In this case, the catalyst may be prepared in the form of cobalt oxide. Catalytically active metal components or promoters may be present in addition to the cobalt, if desired. Examples of suitable Fischer Tropsch promoters include Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Cu, Ag, Au, Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Rh, Pd, Os, Ir, Pt, Mn, B, P, and Re.
The present catalyst preferably contains from 2 to 35% by weight, especially from 5 to 25% by weight, of cobalt, but catalysts wherein the catalytic metal is selected from iron, cobalt, nickel and/or ruthenium are all contemplated. Normally, the catalytic metal is reduced to provide elemental metal (e.g., elemental iron, cobalt, nickel and/or ruthenium) before use. The catalyst must contain a catalytically effective amount of the metal component(s). The amount of catalytic metal present in the catalyst may vary widely. Typically, the catalyst comprises from about 1 to 50% by weight (as the metal) of total supported iron, cobalt, nickel, platinum, rhodium, rhenium, and/or ruthenium per total weight of catalytic metal and support, preferably from about 1 to 30% by weight. Each of the metals can be used individually or in combination, especially cobalt and ruthenium. Of particular note are catalysts comprising from about 10 to 25% by weight (e.g., about 20% by weight) of a combination of cobalt and ruthenium where the ruthenium content is from about 0.001 to about 1 weight %.
In addition, the present catalyst may include one or more additional promoters or modifiers known to those skilled in the art. When the catalytic metal is cobalt, and/or ruthenium, suitable promoters include at least one metal selected from the group consisting platinum and rhenium. The amount of additional promoter, if present, is typically between 0.001 and 1 parts by weight per 100 parts of carrier.
Support materials that are suitable for use with the present invention include, but are not limited to alumina, zirconia, silica, aluminum fluoride, fluorided alumina, bentonite, titania, ceria, zinc oxide, silica-alumina, and molecular sieves. The support may itself have some catalytic activity. By aluminum fluoride is meant at least one of aluminum fluoride (e.g., alpha AlF3, beta AlF3, delta AlF3, eta AlF3, gamma AlF3, kappa AlF3 and/or theta AlF3). Of note are aluminum fluorides, which are primarily alpha AlF3 and/or beta AlF3.
By fluorided alumina is meant a composition comprising aluminum, oxygen and fluorine. The fluoride content of the fluorided alumina can vary over a wide range, from about 0.001% to about 67.8% by weight. Of note are fluorided aluminas containing from 0.001% to about 10% by weight fluorine. The remainder of the fluorided alumina component will include aluminum and oxygen. The composition may also contain a minor amount (compared to aluminum) of silicon, titanium, phosphorus, zirconium and/or magnesium. The support material comprising fluorided aluminas and/or an aluminum fluoride may be prepared by a variety of methods. For example, U.S. Pat. Nos. 4,275,046, 4,902,838 and 5,243,106 disclose the preparation of fluorided alumina by the reaction of alumina with a vaporizable fluorine-containing fluorinating compound. Suitable fluorinating compounds include HF, CCl3F, CCl2F2, CHClF2, CH3CHF2, CCl2FCClF2 and CHF3. U.S. Pat. No. 5,243,106 discloses the preparation of a high purity AlF3 from aluminum sec-butoxide and HF.
Metals can be supported on aluminum fluoride or on fluorided alumina in a variety of ways. For example, U.S. Pat. No. 4,766,260 discloses the preparation of metals such as cobalt on a fluorided alumina support using impregnation techniques to support the metal. U.S. Pat. No. 5,559,069 discloses the preparation of a multiphase catalyst composition comprising various metal fluorides including cobalt fluoride homogeneously dispersed with aluminum fluoride. PCT International Publication No. 97/19751 discloses the preparation of multiphase catalyst compositions comprising metallic ruthenium homogeneously dispersed with various metal fluorides including aluminum fluoride.
Phases of aluminum fluoride such eta, beta, theta and kappa can be prepared as described in U.S. Pat. No. 5,393,509, U.S. Pat. No. 5,417,954 and U.S. Pat. No. 5,460,795.
The catalysts of the present invention may be prepared by methods known to those skilled in the art. These include impregnating the catalytically active compounds or precursors onto a support, extruding one or more catalytically active compounds or precursors together with support material to prepare catalyst extrudates and/or precipitating the catalytically active compounds or precursors onto a support. The most preferred method of preparation may vary, depending for example on the desired catalyst particle size. Those skilled in the art are able to select the most suitable method for a given set of requirements.
One method of preparing a supported metal catalyst (e.g., a supported cobalt catalyst) is by incipient wetness impregnation of the support with an aqueous solution of a soluble metal salt such as nitrate, acetate, acetylacetonate or the like. Another method involves preparing the catalyst from a molten metal salt. For example, the support can be impregnated with a molten metal nitrate (e.g., Co(NO3)2.6H2O). Alternatively, the support can be impregnated with a solution of zero-valent cobalt such as Co2(CO)8, Co4(CO)12, or the like, in a suitable organic solvent (e.g., toluene). The impregnated support is dried and reduced with hydrogen. The hydrogen reduction step may not be necessary if the catalyst is prepared with zero valent cobalt. In another embodiment, the impregnated support is dried, oxidized with air or oxygen and reduced with hydrogen.
Typically, at least part of the metal component(s) of the catalysts of the present invention are present in a reduced state, i.e., metallic state. Therefore, it is normally advantageous to activate the catalyst prior to use by a reduction treatment, in the presence of hydrogen at an elevated temperature. This is typically accomplished by treating the catalyst with hydrogen at a temperature in the range of from about 75 to about 500xc2x0 C., for about 0.5 to about 16 hours at a pressure of about 1 to about 75 atm. Pure hydrogen may be used in the reduction treatment as well as a mixture of hydrogen and an inert gas such as nitrogen. The amount of hydrogen may range from about 1% to about 100% by volume.
The feed gases charged to the invention process must comprise hydrogen or a hydrogen source and carbon monoxide. H2/CO mixtures suitable as a feedstock for conversion to hydrocarbons according to the process of this invention can be obtained from light hydrocarbons such as methane by means of steam reforming or partial oxidation or can alternatively be provided by the gasification of coal. The hydrogen is preferably provided by free hydrogen, although some Fischer-Tropsch catalysts have sufficient water gas shift activity to convert some water to hydrogen for use in the Fischer-Tropsch process. It is preferred that the mole ratio of hydrogen to carbon monoxide in the feed is greater than 1:1. A preferred feed gas stream contains hydrogen and carbon monoxide in a molar ratio of about 2:1. A preferred range of hydrogen to carbon monoxide mole ratios is from 1.0 to 2.5. The feed gas may also contain carbon dioxide. The feed gas stream should contain a low concentration of compounds or elements that have a deleterious effect on the catalyst. Hence, the feed gas may need to be treated to ensure low concentrations of sulfur or nitrogen compounds such as hydrogen sulfide, ammonia and carbonyl sulfides.
The feed gas is contacted with the catalyst in a reaction zone. Mechanical arrangements of conventional design may be employed as the reaction zone. For example, fixed bed, slurry phase, slurry bubble column or ebulliating bed reactors. Accordingly, the size of the catalyst particles may vary depending on the reactor in which they are to be used.
The process of the invention may be performed in a fluid bed or a fixed bed or in a slurry in a liquid e.g. liquid hydrocarbon product. The activation of the catalyst may be performed in the same or a different reactor.
The gas hourly space velocity through the reaction zone may range from about 100 v/hr/v to about 5000 v/hr/v, preferably from about 300 v/hr/v to about 1500 v/hr/v. The reaction zone temperature is in the range from about 160xc2x0 C. to about 300 xc2x0 C. Preferably, the reaction zone is operated at conversion promoting conditions at temperatures from about 190xc2x0 C. to about 260xc2x0 C. The reaction zone pressure is in the range of about 80 psig (653 kPa) to about 1000 psig (6994 kPa), preferably, from 80 psig (653 kPa) to about 600 psig (4237 kPa), more preferably, from about 140 psig (1066 kPa) to about 400 psig (2858 kPa).
The process products will have a great range of molecular weights. Typically, the carbon number range of the product hydrocarbons will start at methane and continue to the limits observable by modem analysis, about 50 to 100 carbons per molecule. Preferably, the product hydrocarbons are primarily paraffins.
The wide range of hydrocarbon species produced in the reaction zone will typically afford liquid phase products at the reaction zone operating conditions. Therefore the effluent stream of the reaction zone will often be a mixed phase stream. The effluent stream of the reaction zone may be cooled to effect the condensation of additional amounts of hydrocarbons and passed into a vapor-liquid separation zone. The vapor phase material may be passed into a second stage of cooling for recovery of additional hydrocarbons. The liquid phase material from the initial vapor-liquid separation zone together with any liquid from a subsequent separation zone may be fed into a fractionation column. Typically, a stripping column is employed first to remove light hydrocarbons such as propane and butane. The remaining hydrocarbons may be passed into a fractionation column wherein they are separated by boiling point range into products such as naphtha, kerosene and fuel oils. Hydrocarbons recovered from the reaction zone and having a boiling point above that of the desired products may be passed into conventional processing equipment such as a hydrocracking zone in order to reduce their molecular weight. The gas phase recovered from the reactor zone effluent stream after hydrocarbon recovery may be partially recycled if it contains a sufficient quantity of hydrogen and/or carbon monoxide.
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following specific embodiments are to be construed as illustrative, and not as constraining the remainder of the disclosure in any way whatsoever.